The world's interest in restrictions on emissions of green house gases such as methane CH4 and carbon dioxide CO2 has been increasing over the years. Methane CH4, in particular, is 21 times more potent than carbon dioxide CO2 in driving the global warming, and therefore emissions of methane CH4 to atmosphere should not be left unnoticed. In the meantime, a large amount of methane CH4, as much as 10 to 40 Nm3 (pure methane) per ton of coal, is being released during coal mining from a coal mine.
Coal mines emit two types of methane CH4 gas: CMM (Coal Mine Methane) gas (with a concentration of about 30 to 50 wt %), existing in and recovered from coal seams through degassing bore holes using vacuum pumps for safety reasons, and VAM (Ventilation Air Methane) gas (with a concentration of 0.3 to 0.7 wt %) released through ventilation from the mine tunnels and the coal face.
Therefore, effective use of the methane contained in the gases emitted from coal mines without releasing it to atmosphere will make significant economic and social contributions.
Patent Document 1 discloses a gas turbine capable of making use of gases with a methane concentration of below the explosion limit, such as landfill gas produced in the landfill, or the gases emitted from coal mines as mentioned above, as fuel.
Patent Document 2 discloses a gas engine for power generation using methane gas emitted from a coal mine as fuel. The gas engine power generating facility will be described with reference to FIG. 4 attached herewith, which shows a sketch of the inside of a coal mine in more detail than FIG. 5 of Patent Document 2.
FIG. 4 is a schematic illustration of a coal mine and a gas engine power generating facility 200 built near the coal mine. In FIG. 4, inside the coal mine are layers of coal seams C0 and the coal seam being mined C1.
A ventilation hole 206 is provided for communicating the inside of the coal mine with the outside.
In the coal face 204 of the coal mine, degassing bore holes 208 are drilled in the coal seam being mined C1, and CMM gas emitted from the bore holes 208 is sent to the gas engine power generating facility 200 through a pipe 210 disposed inside the ventilation hole 206 by means of a vacuum pump 211.
The ventilation air methane gas VAM emitted from the coal mine through the ventilation hole 206 is sent to the gas engine power generating facility 200 through a pipe 212. Electric power E and steam S generated through operation of the gas engine power generating facility 200 are sent to a utility facility 202 in the mine premises or further to other consumers.
Patent Document 1: Japanese Patent Application Publication No. 2010-19247
Patent Document 2: US Patent Application Publication No. 2005/0205022 (Description)
Gas engines that use methane gas as fuel are an internal combustion engine expected to be used widely in future because of their advantage that they emit only water H2O and carbon dioxide CO2 as combustion products and thus cause very little pollution to the environment.
In an engine using a low energy gas such as methane as fuel, the amount of fuel supplied to the respective cylinders from fuel gas supply solenoid valves is insufficient, and the shortage needs to be complemented through the intake system. Complementing this shortage of fuel gas before a turbocharger is a common practice referred to as pre-mixing before a turbocharger.
Pre-mixed gas mixture containing fuel gas mixed therein is compressed by the turbocharger, after which it is cooled by an intercooler so that water vapor contained in the intake gas condenses into water drops. Part of the pre-mixed gas mixture is released as drain gas with the water drops when the water is discharged.
However, as noted above, methane CH4 is 21 times more potent than carbon dioxide CO2 in driving the global warming. Also, discharging compressed pre-mixed gas mixture leads to a decrease in volumetric efficiency and thermal efficiency of the internal combustion engine.